It is ultimately the PIC who has to say STOP if a situation like this develops. Unfortunately he didn't.
I see nothing in the AAIB report that convinces me that Andrew Walker thought he was PIC.
1. He hadn't flown for more than three months, and had only flown 3hr 45mins in the last thirteen months. He had planned that day to fly with the club as P/UT carrying out refresher training when the CFI asked him instead to fly to Exeter. He correctly responded that he was unqualified to undertake that flight, but was told by the CFI he would be accompanied by a more experienced pilot. We are not told who signed the aeroplane out, and the AAIB were unable to establish whether Mr Walker knew that the accompanying pilot was not an instructor ... but I could well believe that he considered this to be an alternative sortie for his refresher training.
2. The only evidence the AAIB advance for considering him to be PIC is:
"The relatively inexperienced pilot was paying for the fuel for the two flights, the purpose of which was for him to build up his flight hours. He was seated in the left seat and he was almost certainly handling the controls throughout the accident flight. Thus he should be considered the pilot-in-command ...." Well sorry, AAIB, but that rationale is nonsense ... on that basis every student pilot would be PIC on every flight!
3. We are being asked to believe that a young pilot sufficiently committed to be working towards an ATPL carried out this flight believing he was PIC whilst showing no evidence whatsoever of command decision making - the accompanying pilot appears alone (or in conjunction with others but not Mr Walker) to have decided on fuel (and therefore W&B) at both Blackpool and Exeter. We are also asked to believe that, despite demonstrating he was conscientious about his lack of currency, and with 1.3 hours of total instrument flying experience, he took off from Blackpool into a 200 ft overcast on the outward journey, and departed Exeter on the return flight knowing he would arrive in poor weather and fading daylight, all whilst believing he was PIC.
A tragic and entirely avoidable accident - certainly. But we may never know whether Andrew Walker believed he was the pilot in command and was therefore guilty of causing the accident through his poor decision making. Or whether, as seems at least as likely to me, he was merely guilty of not establishing that his assumed P/UT status was factual. If the latter, his 'offence' is one that is regularly repeated around the country by pilots, student and qualified, when undertaking training flights.